Mask and substrate alignment for solder bump process

ABSTRACT

A system for of aligning a mask to a substrate comprising: a fixture for holding the mask and the substrate in fixed positions relative to each other; means for holding the substrate, the means for holding the substrate protruding through openings in a table and the fixture, the means for holding fixedly mounted on a stage, the stage moveable in first and second directions and rotatable about an axis relative to the table; means for affixing the fixture containing the mask and the substrate to the table; means for controlling the means for temporarily affixing so as to generate a uniform force around a perimeter of the fixture to effectuate the temporarily affixing; means for aligning the mask to the substrate, the means for aligning controlling movement of the stage in the first and second directions and rotation about the axis; and means for fastening the fixture together.

This application is a continuation of copending U.S. patent applicationSer. No. 10/604,142 filed on Jun. 27, 2003.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor processing;more specifically, it relates to an apparatus and method for aligning asolder bump mask to a substrate.

BACKGROUND OF THE INVENTION

The formation of solder bumps or controlled collapse chip connection(C4) interconnects on semiconductor substrates requires assembly of analignment fixture holding the substrate and a metal mask having holesthrough which the solder bump processes of sputter clean, pad limitingmetallurgy evaporation and solder bump evaporation are performed. Priorto these process steps, the mask must be aligned to the substrate.Traditionally, alignment of mask to substrate has been done manually,however as solder bump sizes and spacing between solder bumps hasdecreased; manual alignment has been shown to be unable to provide thealignment accuracy needed.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a system for of aligning amask to a substrate comprising: an alignment fixture for temporarilyholding the mask and the substrate in fixed positions relative to eachother; means for holding the substrate by a bottom surface, the meansfor holding the substrate protruding through an opening in a table andan opening in the alignment fixture, the means for holding fixedlymounted on a stage assembly, the stage assembly moveable in first andsecond directions and rotatable about an axis relative to the table;means for temporarily affixing the alignment fixture containing the maskand the substrate to the table; means for controlling the means fortemporarily affixing so as to generate a uniform force around aperimeter of the alignment fixture to effectuate the temporarilyaffixing; means for aligning the mask to the substrate, the means foraligning controlling movement of the stage assembly in the first andsecond directions and rotation about the axis; and means for temporarilyfastening the alignment fixture together.

A second aspect of the present invention is a method for of aligning amask to a substrate comprising in the order recited: (a) placing abottom ring of an alignment fixture on an alignment tool; (b) loading asubstrate onto a chuck; (c) se curing the substrate on the chuck; (d)locating alignment targets on the substrate relative to fixed positionsof a first X-Y stage and a rotational stage mounted on the first X-Ystage; (e) placing the mask on the bottom ring and placing a top ring ofthe alignment fixture on the mask, the top ring aligned to the bottomring; (f) applying a clamping force of a first predetermined amount offorce to the alignment fixture sufficient to prevent the mask frommoving relative to the top and bottom rings; (g) locating alignmentmarks on the mask relative to a fixed position of a second X-Y-stage,the first X-Y stage and the table mounted to the second X-Y stage, X andY orthogonal displacement directions associated with each of the firstand second X-Y stages being co-aligned; (h) calculating an X distance inthe X direction and a Y distance in the Y direction to move the firstX-Y stage and an angle to rotate the rotational stage through in orderto align the alignment marks to the alignment targets; (i) increasingthe applied clamping force to a second predetermined amount of force,releasing the substrate from the chuck, and increasing the appliedclamping force to a third predetermined amount of force; (j) temporarilyfastening the alignment fixture containing the mask and the substratetogether; and (k) releasing the applied clamping force.

A third aspect of the present invention is a method for of aligning amask to a substrate comprising in the order recited: (a) providing analignment fixture for temporarily holding the mask and the substrate infixed positions relative to each other; (b) providing an alignment toolincluding a stage assembly and a table; (c) placing a bottom ring of thealignment fixture on the table; (d) securing the substrate on the chuckand locating alignment targets on the substrate relative to a fixedposition of the stage assembly; (e) placing the mask on the bottom ringand placing a top ring of the alignment fixture on the mask, the topring aligned to the bottom ring; (f) applying a affixing force of afirst predetermined amount of force to the alignment fixture sufficientto prevent the mask from moving; (g) locating alignment marks on themask relative to a fixed position of the stage assembly; (h) moving thesubstrate relative to the mask in order to align the alignment marks tothe alignment targets; (i) increasing the applied affixing force to asecond predetermined amount of force, releasing the substrate from thechuck, increasing the applied affixing force to a third predeterminedamount of force; (j) temporarily fastening the alignment fixturecontaining the mask and the substrate together; and (k) releasing theapplied affixing force.

BRIEF DESCRIPTION OF DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of an illustrative embodiment when readin conjunction with the accompanying drawings, wherein:

FIG. 1A is a top view of a bottom ring of a substrate to mask alignmentfixture for forming interconnects according to the present invention;

FIG. 1B is a cross-section view through line 1B-1B of FIG. 1A;

FIG. 2 is a top view of an evaporative mask for forming interconnectsaccording to the present invention;

FIG. 3A is a top view of a top ring of the substrate to mask alignmentfixture for forming interconnects according to the present invention;

FIG. 3B is a cross-section view through line 3B-3B of FIG. 3A;

FIG. 4A is a partial cross-section view through the assembled substrateto mask alignment fixture for forming interconnects according to thepresent invention;

FIG. 4B is a top view and FIG. 4C is a side view of a spring clipillustrated in FIG. 4A;

FIG. 5 is a top view of an alignment tool according to the presentinvention;

FIG. 6 is a cross-section view through line 6-6 of FIG. 5;

FIG. 7A is a cross-section view through an alignment pin mechanismaccording to the present invention;

FIG. 7B is a diagram illustrating the tolerances between the alignmentpin of FIG. 7A and a mask;

FIG. 8 is a side view of a clamping mechanism according to the presentinvention;

FIG. 9 is a side view of a clipping mechanism, according to the presentinvention;

FIG. 10A is a top view of a substrate having alignment marks accordingto the present invention;

FIG. 10B is a top view of a mask having alignment marks according to thepresent invention;

FIG. 10C is a top view of the substrate and mask alignments are theywould appear in perfect alignment;

FIG. 11A is a diagram of initial wafer alignment fiducials prior to maskto wafer alignment according to the present invention;

FIG. 11B is a diagram of initial mask alignment fiducials coordinatesprior to mask to wafer alignment according to the present invention; and

FIG. 12 is a flowchart of the method for aligning a substrate to a maskaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention the term interconnect isdefined as a solder bump or C4 interconnection that is formed byevaporation onto a substrate through holes formed in a mask. The termsubstrate is defined to include but is not limited to semiconductorsubstrates (or wafers) including bulk silicon substrates andsilicon-on-insulator (SOI) substrates. The term mask includes but is notlimited to metal masks fabricated from molybdenum or other metals.Movement in any of the mutually orthogonal X, Y and Z directions and therotational Θ direction (as described infra) includes movement in bothpositive and negative directions.

FIG. 1A is a top view of a bottom ring 100 of a substrate to maskalignment fixture for forming interconnects according to the presentinvention and FIG. 1B is a cross-section view through line 1B-1B of FIG.1A. In FIGS. 1A and 1B, bottom ring 100 includes an outer lip 105 and aninner lip 110 joined by an integral plate 115. Inner lip 110 defines theextent of an opening 120 centered in bottom ring 100. Plate 115 includesa multiplicity of openings 125. Opening 120 provides access for asubstrate positioning chuck (see FIG. 6) and openings 125 are forthermal expansion and heat retention control during evaporativeprocesses and to make bottom ring 100 lighter. The difference in heightbetween outer lip 105 and inner lip 110 is H1 In one example, for astandard 200 mm diameter wafer about 650 microns thick, H1 is about0.007 inches. Bottom ring 100 also includes an alignment pin hole 130Aand a diametrically opposed alignment pin slot 130B, each positionedadjacent to an outer perimeter 135 of the bottom ring. Bottom ring 100further includes a multiplicity (in the present example 6) of retainerpost holes 140 evenly spaced about and adjacent to outer perimeter 135of the bottom ring.

FIG. 2 is a top view of an evaporative mask for forming interconnectsaccording to the present invention. In FIG. 2, a circular mask 145includes a multiplicity of openings 150 arranged in groups 155. Eachgroup 155 corresponds to a chip on a substrate (not illustrated) thatwill be placed under mask 145 as illustrated in FIG. 4 and describedinfra. Mask 145 also includes an alignment pin hole 160A and adiametrically opposed alignment pin slot 160B, each positioned adjacentto an outer perimeter 160 of the mask. Mask 145 further includes amultiplicity (in the present example 6) of retainer post holes 165evenly spaced about and adjacent to outer perimeter 160 of the mask.

FIG. 3A is a top view of a top ring 175 of the substrate to maskalignment fixture for forming interconnects according to the presentinvention and FIG. 3B is a cross-section view through line 3B-3B of FIG.3A. In FIGS. 3A and 3B, top ring 175 has a lower lip 180 protruding froma flange 185. Lower lip 180 protrudes a distance H2. In one example, fora standard 200 mm diameter wafer having a thickness of about 725microns, H2 is about 0.002 inches. Top ring 175 includes an opening 190centered within top ring 175. Top ring 175 also includes an alignmentpin hole 195A and a diametrically opposed alignment pin slot 19513, eachpositioned adjacent to an outer perimeter 200 of the mask. Top ring 175further includes a multiplicity (in the present example 6) of retainerposts 205 evenly spaced about and adjacent to outer perimeter 200 of themask.

FIG. 4A is a partial cross-section view through an assembled substrateto mask alignment fixture 210 for forming interconnects according to thepresent invention. In FIG. 4A, only a portion of assembled fixture 210is illustrated. Substrate 215 and mask 145 are illustrated contained inalignment fixture 210. Retainer posts 205 protrude through retainer postholes 140 in bottom ring 100 and pass through retainer post holes 165 inmask 145. When spring clips 220 are slid onto retainer posts 205perimeter 160 of mask 145 is held in a slightly pressed down position bylower lip 180 of top ring 175 against outer lip 105 of spring clips 220thus holding substrate 215, mask 145, top ring 175 and bottom ring 100together. Spring clips 220 are not put in place until mask 145 isaligned to substrate 215.

The combination of the difference in heights between outer and innerlips 105 and 110 of bottom ring 100 and lower lip 180 of top ring 175deflects (or bows) substrate 215 and mask 140 into very shallow butsemi-spherical shape by pressing perimeter 160 of mask 145 and perimeter225 of substrate 215 towards bottom ring 100.

Since alignment fixture 210 is mounted in a dome of a metal evaporator,the bow imparted to substrate 215 prevents or reduces such problemsassociated with evaporation through an knife edge opening in a mask suchas sputter haze, PLM flaring and solder pad haloing.

FIG. 4B is a top view and FIG. 4C is a side view of spring clip 220illustrated in FIG. 4A. Spring clip 220 includes a notch 230 thatengages a lower end 235 of retainer post 205 as illustrated in FIG. 4A.Spring clip 220 also includes a retraction hole 240 to enable removal ofspring clips 205 and thus disassembly of alignment fixture 210 (see FIG.4A). FIG. 4C illustrates spring clip 220 before engagement withretaining post 205.

FIG. 5 is a top view of an alignment tool 250 according to the presentinvention. In FIG. 5, alignment tool 250 includes a top plate 255 achuck 260, alignment pin mechanisms (illustrated in FIG. 8A anddescribed more fully infra), a multiplicity of clamping mechanisms 270(illustrated in FIG. 9 and described more fully infra) and amultiplicity of clipping mechanisms 275 (illustrated in FIG. 10 anddescribed more fully infra). Chuck 260 extends through an opening 280 intop plate 255 and includes a multiplicity of O-rings 285. Each O-ringsurrounds a vacuum port 290 centered within the ring. O-rings 285 arearranged in a ring and located adjacent to a perimeter 295 of chuck 260.Clamping mechanisms 270 are evenly spaced around a locator ring 300centered on chuck 260 that roughly defines the position occupied byalignment fixture 210. Clipping mechanisms 275 are evenly spaced aroundlocator ring 300. Alignment pin mechanisms (containing alignment pins305) are positioned diametrically opposed adjacent to and interior oflocator ring 300. Attached to an underside of top plate 255 is a fixedinner actuator ring 310 having outwardly protruding spokes 315. Spokes315 extend through eccentric slots 350 (not shown in FIG. 5, see FIG. 6)in a rotatable outer actuator ring 320. Outer actuator ring 320 can moveup and down in the Z direction (see FIG. 6) as well as rotate in the θdirection (see FIG. 6) about an axis defined by the Z direction.

While six clamping mechanisms 270 and six clipping mechanisms 275 areillustrated in FIG. 5, any number greater than or equal to threemechanisms of each type may be used. While two alignment pins mechanismsare illustrated in FIG. 5, a greater number of alignment pin mechanismsmay be employed in which case the number and arrangement of alignmentpin holes 130A and 195A and alignment pin slots 130B and 195B (see FIGS.1A and 3A respectively) will change.

While eight O-rings 285 are illustrated in FIG. 5, chuck 260 may includea lesser or greater number of O-rings, the minimum number being three.The inventors have discovered that chucks using a conventional singleO-ring design can induce errors up to 10 times greater then the accuracyof the encoders because of O-ring distortion during the alignmentprocess. This distortion is caused by the fact that mask 145 (see FIG.2) and substrate 215 (see FIG. 4A) are in slight frictional contact thatcan cause a single O-ring to distort or creep. The radially orientatedmulti O-ring design of chuck 260 all but eliminates translation errorscaused by O-ring creep. Note, a single O-ring design using a hardO-ring, presents other problems such as substrate breakage, since beingless flexible there is insufficient compressibility in the O-ring toabsorb acceleration induced shock.

FIG. 6 is a cross-section view through line 6-6 of FIG. 5. In FIG. 6,chuck 260 is mounted on an rotational stage 325 for θ adjustment that inturn is mounted on an upper X-Y stage 330 for X direction and Ydirection (the Y direction is into plane of the paper) adjustment of theposition of substrate 215. Upper X-Y stage 330 is in turn mounted on alower X-Y stage 340. It is preferred, but not necessary that the Xmovement of upper X-Y stage 330 is perpendicular to the Y movement oflower X-Y stage 340, the Y movement of upper X-Y stage 330 isperpendicular to the X movement of lower X-Y stage 340 and the topsurfaces 325A, 330A and 340A respectively of rotational stage 325, upperX-Y stage 330 and lower X-Y stage 340 are parallel. Top plate 255 isalso mounted on lower X-Y stage 340 via brackets 345. Since alignmentpin mechanism 265 is fixed to top plate 255, a measurable and repeatablerelationship exists between alignment pins 305 and chuck 260 as long asthe relative positions of upper X-Y stage 325 and lower X-Y stage 340are known.

As outer actuator ring 320 rotates spokes 315 fixed to inner actuatorring 310 and passing through slanted slots 350 in the outer actuatorring cause the outer actuator ring to translate in the Z direction.Note, the X, Y and Z directions are orthogonal to each other. A lip 355attached to outer actuator ring 320 thus also moves in the Z direction.A push rod of clamping mechanism 270 rides on lip 355 (see FIG. 8) soclamping is controlled by the rotation of outer actuator ring 315 andclamping force is uniformly applied by all clamping mechanisms 275 (seeFIG. 5). Alignment tool 250 also includes an optical system 360(generally a lens and a camera) linked to a computer 365 containingpattern recognition software as well a software for controlling movementrotational stage 325, upper X-Y stage 330 and lower X-Y stage 340. Thepattern recognition software detects the position of alignment targetson substrate 215 and alignment marks on mask 145. Computer 365 is linkedto stepping motors on rotational stage 325, upper X-Y stage 330 andlower X-Y stage 340 for controlling movement of substrate 215. Computer365 calculates the amount of upper X-Y stage 330, lower X-Y stage 340and rotational stage 325 movement required to align mask 145 with thesubstrate 215.

During the alignment process, it is important that movement of substrate215 is precise and accurate. In one example, encoders within rotationalstage 325 are accurate to 0.001 degree encoders within upper X-Y stage330 and lower X-Y stage 340 are accurate to 1 micron.

FIG. 7A is a cross-section view through alignment pin mechanism 265according to the present invention. In FIG. 7, alignment pin mechanism265 includes a body 370 having a lower chamber 375 open to an upperchamber 380. Alignment pin 305 includes an upper narrow portion 385 anda wide lower portion 390. Alignment pin 305 extends through upperchamber 380 into lower chamber 375. Alignment pin 305 is restricted inmovement in the X and Y directions by sleeve bearing 395 and is moveablein the Z direction due to spring 400 contained within lower chamber 375.Lower portion 390 of pin 305 passes through alignment pin hole 130A (oralignment pin slot 130B) in bottom ring 100 as well as an opening 405 intop plate 255. Upper portion 385 of pin 305 passes through alignment pinhole 160A (or alignment pin slot 160B) in mask 145. Upper portion 385 ofpin 305 also passes through alignment pin hole 195A (or alignment pinslot 195B) in top ring 175.

FIG. 7B is a diagram illustrating the tolerances between alignment pin305 of FIG. 7A and mask 145. In FIG. 7B, upper portion 385 of alignmentpin 305 has a diameter of D1. Lower portion of alignment pin 305 hasdiameters D2. Alignment pin hole 160A of mask 145 has a diameter of D3.Note alignment pin slot 160B (see FIG. 2) has a width of D3 and is about2D3 long. D2 is greater than D1. In one example D2−D1=0.010 inch andD3−D1=0.002 inch.

Returning to FIG. 7A, during alignment of mask 145 to a substrate 215,it is important that the mask does not move. Mask 145 experiences forcesin the X, Y and θ directions. Alignment pins 305 restrict this movement.It is also important that alignment pins 305 move freely in the Zdirection. Spring 400 ensures that there is always a net upward force(positive Z direction) on alignment pin 305 to resist downward force(negative Z direction) imparted to mask 145 by clamping mechanisms 270(see FIG. 5) during the alignment process to keep constant contactbetween the mask and bottom ring 100.

FIG. 8 is a side view of clamping mechanism 270 according to the presentinvention. In FIG. 8, clamping mechanism 270 includes a mounting bracket410 mounted to top plate 255, a body 415, a moveable clamp finger 420slidably mounted in body 415 and a push rod 430 that operably engageslip 355 of outer actuator ring 320. Clamp finger 420 can be slid over orretracted from top ring 175 by a mechanism not illustrated. As outeractuator ring rotates 320, because spokes 315 are fixed to inneractuator ring 310 and extend through eccentric slots 350, lip 355 movesup or down depending on the direction of rotation of outer actuator ring320. Push rod 430, engaged on lip 355, moves up and down with lip 355,causing clamp finger 420 to apply pressure to the assembled alignmentfixture 210 comprising bottom ring 100, substrate 215, mask 145 and topring 175. Clamp finger 420 is spring loaded so movement of lip 355toward top plate 255 works against the spring and moves clamp finger 420away from alignment fixture 210. As lip 355 lowers, increasing pressureis applied to alignment fixture 210.

Once the alignment process is completed, it is important that theclamping process be uniform across alignment fixture 210, smooth andreproducible time to time. Any non-uniformity will result sidewayslippage of mask 145 and/or substrate 215 and thus misalignment afterthe alignment process. Any non-smoothness in clamping can result in ashock that can cause mask 145 and/or substrate 215 movement, againresulting in misalignment after the alignment process.Non-reproducibility in clamping pressure can result in clipping (seeFIG. 9 and discussion infra) problems. Since all clamping mechanisms 270are driven by outer actuator ring 320 clamping is uniform and smooth. Aprecision drive mechanism (not shown) driving outer actuator ring 320ensures reproducible and precision controlled clamping pressure.

FIG. 9 is a side view of clipping mechanism 275 according to the presentinvention. In FIG. 9, clipping mechanism 275 includes a base 435 mountedto top plate 255, a slide 440 having a slot (not shown) to hold a clip220, and a roller 445 mounted to a support 450 attached to base 435. Inoperation, a clip 220 placed on slide 440. As slide 440 is pushedforward toward retaining post 235, clip 220 is compressed by roller 445.As slide 440 continues forward, clip 220 engages retaining post 235 andbecomes released from roller 445 allowing clip 220 to “spring” open.When slide 440 is retracted, clip 220 remains in place due to frictionforces between the clip and bottom ring 100.

It is important that insertion of clips 220 do move top ring 175 towhich retainer posts 235 are fixedly attached. Movement of top ring 175will cause mask 145 to move, thus changing the alignment of mask 145 tosubstrate 215. Clipping mechanism 275 “preloads” clips 220 so thatforces in the Z direction are eliminated during insertion, resulting inminimal X direction and Y direction forces being applied to retainingpost 235 and top ring 175 during insertion. Each slide 440 moved toengage retaining posts 235 simultaneously by a mechanical mechanism.

FIG. 10A is a top view of a substrate having alignment marks accordingto the present invention. In FIG. 10A, substrate 100 includesdiametrically opposed (or nearly diametrically opposed) left and rightcourse alignment targets 455A and 455B respectively and diametrically(or nearly diametrically opposed) left and right fine alignment targetsets 460A and 460B respectively. Left and right course alignment targets455A and 455B are used by pattern recognition software residing oncomputer 365 (see FIG. 6) during alignment operations. Left and rightfine alignment targets 460A and 460B are used by an operator to checkthe quality of alignment operations.

FIG. 10B is a top view of a mask having alignment marks according to thepresent invention. In FIG. 10B, mask 145 includes diametrically opposed(or nearly diametrically opposed) left and right course alignment marks465A and 465B respectively and diametrically (or nearly diametricallyopposed) left and right fine alignment mark sets 470A and 470Brespectively. Left and right course alignment targets 465A and 465B areused by pattern recognition software residing on computer 365 (see FIG.6) during alignment operations. Left and right fine alignment targets470A and 470B are used by an operator to check the quality of alignmentoperations. Also illustrated in FIG. 10B are alignment pin hole 160A andalignment pin slot 160B.

FIG. 10C is a top view of the substrate and mask alignments are theywould appear in perfect alignment. In FIG. 10C, course substratealignment target 455A (455B) is centered on the middle two course maskalignment marks 465A (465B). All mask fine alignment marks 470A (470B)are centered in substrate fine alignment targets 460A (460B).

FIG. 11A is a diagram of initial wafer alignment fiducial coordinatesprior to mask to wafer alignment according to the present invention. InFIG. 11A, rotational stage in 325 can move the Θ direction and upper X-Ystage 330 can move in the X and Z direction. A reference location 475has upper X-Y stage coordinates X₀ and Y₀ and rotational stage 325coordinate Θ₀. Rotational stage 325 and upper X-Y stage 330 areinitially moved to coordinates X₀, Y₀ and Θ₀ respectively (as describedinfra in reference to FIG. 12) and all subsequent motions of rotationalstage 325 and upper X-Y stage 330 stage motions are referenced tocoordinates X₀, Y₀ and Θ₀ respectively. Wafer 215 includes a leftfiducial mark 480 containing left course alignment target 455A (see FIG.10A) and fine alignment targets 460A (see FIG. 10A) and a right fiducialmark 485 containing right course alignment target 455B (see FIG. 10A)and fine alignment targets 460B (see FIG. 10A). When rotational stage325 and upper X-Y stage 330 are moved to coordinates X₀, Y₀ and Θ₀respectively, left fiducial 480 is at location X_(LW0), Y_(LW0) andΘ_(LW0) and right fiducial 485 is at location X_(RW0), Y_(RW0) andΘ_(RW0).

FIG. 11B is a diagram of initial mask alignment fiducial coordinatesprior to mask to wafer alignment according to the present invention. InFIG. 11B, lower X-Y stage 340 can move in the X and Z direction. Lowerstage X-Y 340 is referenced to reference position 475. All subsequentmotions of lower X-Y stage 340 are referenced to coordinates X₀ and Y₀.As long as the X movement of upper X-Y stage 330 is perpendicular to theY movement of lower X-Y stage 340, the Y movement of upper X-Y stage 330is perpendicular to the X movement of lower X-Y stage 340 and the topsurfaces of rotational stage 325, upper X-Y stage 330 and lower X-Ystage 340 are parallel. Mask 145 includes a left fiducial mark 490containing left course alignment mark 465A (see FIG. 10B) and finealignment marks 470A (see FIG. 10B) and a right fiducial mark 495containing right course alignment mark 465B (see FIG. 10A) and finealignment marks 470B (see FIG. 10B). When lower X-Y stage 330 isreferenced to coordinates X₀ and Y₀, left fiducial 490 is at locationX_(LM0) and Y_(LM0) and right fiducial 495 is at location X_(RM0) andY_(RM0).

Reference to FIGS. 11A and 11B will be useful in understanding theprocess illustrated in FIG. 12 and described infra. In FIG. 12 it isassumed that the X movement of upper X-Y stage 330 is perpendicular tothe Y movement of lower X-Y stage 340, the Y movement of upper X-Y stage330 is perpendicular to the X movement of lower X-Y stage 340 and thetop surfaces of rotational stage 325, upper X-Y stage 330 and lower X-Ystage 340 are parallel. Any deviations from these conditions, requiresmore complex calculations then described infra in reference to FIG. 12.

FIG. 12 is a flowchart of the method for aligning a substrate to a maskaccording to the present invention In step 500, a bottom ring of thealignment fixture is loaded onto the alignment tool and in step 505 asubstrate is placed on the chuck of the alignment tool and vacuumapplied to the chuck, holding the substrate fast to the chuck.

In step 510, the course substrate alignment targets are located. First,the upper X-Y and rotational stages are moved to a predetermined X₀, Y₀and θ₀ positions. For the substrate left course alignment target thepattern recognition software locates the center of the left coursealignment target and moves the upper X-Y stage and the rotational stageso the center of the left course alignment target is aligned with thecenter of the optical system. The pattern recognition software againlocates the center of the substrate left course alignment target andmoves the upper X-Y stage and the rotational stage so the center of theleft course alignment target is aligned with the center of the opticalsystem. The double locating brings the substrate left course alignmenttarget to the area of least distortion within the optical system. Thisfixes the substrate left course target starting position on the upperX-Y stage and the rotational stage as X_(LW0), Y_(LW0) and θ_(LW0).

For the substrate right course alignment target the pattern recognitionsoftware locates the center of the right course alignment target andmoves the upper X-Y stage and the rotational stage so the center of theright course alignment target is aligned with the center of the opticalsystem. The pattern recognition software again locates the center of thesubstrate right course alignment target and moves on the upper X-Y stageand the rotational stage so the center of the right course alignmenttarget is aligned with the center of the optical system. The doublelocating brings the substrate right course alignment target to the areaof least distortion within the optical system. This fixes the substrateright course target starting position on the upper X-Y stage and therotational stage as X_(RW0), Y_(RW0) and θ_(RW0).

Next, in step 515, the upper X-Y stage and the rotational stage aremoved fixed distances D_(X), D_(Y) and angle D_(θ). This motioncompensates for the slack that is inherent in the stage mechanics.

In step 520, the mask is positioned (using the alignment pins) over thesubstrate and in step 525 the top ring of the alignment fixture ispositioned (using the alignment pins).

In step 530, a light clamping pressure is applied by the clampingmechanisms in order to prevent motion of the mask during subsequentupper X-Y stage, rotational stage and lower X-Y stage movements. Thepressure applied is just sufficient to bring the mask and top ring ofthe alignment fixture into contact.

In step 535 the upper X-Y stage and the rotational stage are moved adistance −D_(X)/2, −D_(Y)/2 and angle −D_(θ)/2 (i.e. halfway back to X₀,Y₀ and θ₀). This motion compensates for the slack that is inherentbetween the mask and the alignment pins and places the mask in a stableposition.

In step 540, the pattern recognition software locates the center of themask left course alignment mark and moves the lower X-Y stage so thecenter of the left course alignment mark is aligned with the center ofthe optical system. The pattern recognition software again locates thecenter of the mask left course alignment mark and moves the lower X-Ystage so the center of the left course alignment mark is aligned withthe center of the optical system. The double locating brings the maskleft course alignment mark to the area of least distortion within theoptical system. This fixes the left course mark starting position on thelower X-Y stage as X_(LM0) and Y_(LM0).

For the mask right course alignment mark the pattern recognitionsoftware locates the center of the right course alignment mark and movesthe lower X-Y stage so the center of the right course alignment mark isaligned with the center of the optical system (generally a lens and acamera). The pattern recognition software again locates the center ofthe mask right course alignment mark and moves the lower X-Y stage sothe center of the right course alignment mark is aligned with the centerof the optical system. The double locating brings the mask right coursealignment mark to the area of least distortion within the opticalsystem. This fixes the right course mark starting position on lower X-Ystage as X_(RM0) and Y_(RM0).

In step 545, first the rotational displacement of the substrateθ_(W)=tan⁻¹ ((Y_(LW0)−Y_(RW0))/(X_(LW0)−X_(RW0))) and the rotationaldisplacement of the mask θ_(M)=tan⁻¹((Y_(LM0)−Y_(RM0))/(X_(LM0)−X_(RM0))) are calculated. Second, therelative rotational displacement (and correcting theta displacement)between the mask and substrate Δθ_(MW)=θ_(M)−θ_(W) is calculated. Inorder to align the substrate to the mask in the X and Y directionscompensation for the applied rotation is necessary. Third, theX-translation, X′=X_(WL0) cos Δθ_(MW)−Y_(WL0) sin Δθ_(MW) (or X′=X_(WR0)cos Δθ_(MW)−Y_(WR0) sin Δθ_(MW)) and the Y-translation Y′=X_(WL0) sinΔθ_(MW)+Y_(WL0) cos Δθ_(MW) (or Y_(R)′=X_(WR0) sin θ_(MW)+Y_(WR0) COSθ_(MW)) are calculated. Fourth, the correcting displacementsΔY=Y_(ML0)−Y′ or (ΔY=Y_(MR0)−Y′) and ΔX=X_(ML0)−X′ or (ΔX=X_(MR0)−X′)are calculated. Fifth, the Y_(ALIGN)=ΔY+(−D_(Y)/2),X_(ALIGN)=ΔX+(−D_(X)/2) and θ_(ALIGN)=Δθ_(MW)+(−D_(θ)/2) movements ofthe upper X-Y and rotational stages are calculated.

In step 550, the mask and substrate are aligned by moving the upper X-Ystage distances X_(ALIGN) and Y_(ALIGN) and moving the rotational stageangle θ_(ALIGN).

In step 555, the clamping mechanism fully compresses the top ring, maskand substrate against the bottom ring. The chuck vacuum is released atabout 75% full compression in order to avoid breaking the substrate,which is now slightly bowed by the clamping and could be further shockedby air entering the chuck when the substrate releases from the chuckunder the condition of the clamping pressure being greater thanatmospheric pressure.

In step 560, clipping mechanisms install the clips on the retainingposts which keep the bottom ring, substrate, mask and top ring stackunder compression and in alignment.

In step 565, the fine alignment targets/marks are inspected to thealignment of mask to substrate is within specification.

Using the alignment fixture, alignment tool andalignment/clamping/clipping procedure described supra, the inventorshave been able to achieve alignments between mask and substrate on 200mm wafers of better than 20 microns.

The description of the embodiments of the present invention is givenabove for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.Therefore, it is intended that the following claims cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

1. A system for of aligning a mask to a substrate comprising: analignment fixture for temporarily holding said mask and said substratein fixed positions relative to each other; means for holding saidsubstrate by a bottom surface, said means for holding said substrateprotruding through an opening in a table and an opening in saidalignment fixture, said means for holding fixedly mounted on a stageassembly, said stage assembly moveable in first and second directionsand rotatable about an axis relative to said table; means fortemporarily affixing said alignment fixture containing said mask andsaid substrate to said table; means for controlling said means fortemporarily affixing so as to generate a uniform force around aperimeter of said alignment fixture to effectuate said temporarilyaffixing; means for aligning said mask to said substrate, said means foraligning controlling movement of said stage assembly in said first andsecond directions and rotation about said axis; and means fortemporarily fastening said alignment fixture together.
 2. The system ofclaim, wherein said first and second directions are orthogonal to eachother and parallel to said table.
 3. The system of claim 1, wherein saidaxis is perpendicular to said table.
 4. The system of claim 1, furtherincluding an additional stage assembly moveable in said first and seconddirections, said stage assembly and said table mounted on saidadditional stage assembly, movement of said additional stage assembly insaid first and second directions controlled by said means for aligning.5. The system of claim 1, wherein said means for holding said substrateis a vacuum chuck including a circular array of O-rings adjacent to aperimeter of said chuck, each O-ring extending above a top surface ofsaid chuck and surrounding a vacuum port.
 6. The system of claim 5,further including means for releasing vacuum pressure applied to saidvacuum chuck when said uniform force reaches a predetermined value. 7.The system of claim 1, further including at least two mask alignment pinmechanisms, said mask alignment pin mechanisms mounted to said table andcontaining alignment pins, said alignment pins passing through alignmentpin holes in said table and alignment pin holes in said alignmentfixture and engaging mask alignment holes in said mask.
 8. The system ofclaim 7, wherein said alignment pins are spring loaded and can move in athird direction perpendicular to said table.
 9. The system of claim 7,wherein said mask alignment holes comprise a circular alignment hole anda slot.
 10. The system of claim 7, wherein said mask alignment holes arediametrically opposed.
 11. The system of claim 1, wherein said means fortemporarily affixing are two or more clamping mechanisms uniformlyspaced around a perimeter of said alignment fixture.
 12. The system ofclaim 11, wherein said clamping mechanisms further include clampingfingers for compressing and clamping said alignment fixture and pushrods for moving said clamping fingers.
 13. The system of claim 12,wherein said push rods are simultaneously activated by a rotatable ringmoveable in a third direction perpendicular to said table as saidrotatable ring is rotated.
 14. The system of claim 1, wherein said meansfor temporarily fastening insert removable spring clips onto lowerportions of retaining posts extending below a bottom ring of saidalignment fixture, said retaining posts fixed in and extending from atop ring portion of said alignment fixture and through retaining postholes in said mask and retaining post holes in said bottom ring.
 15. Thesystem of claim 14, wherein said means for temporarily fasteningsimultaneously insert said removable spring clips onto said retainingposts.
 16. The system of claim 15, wherein said means for temporarilyfastening compress said removable spring clips prior to insertion ofsaid removable spring clips onto said retaining posts.
 17. The system ofclaim 1, wherein said means for aligning comprises a pattern recognitionsystem including a camera and a computer, said pattern recognitionsystem for determining locations of center points of alignment targetson said substrate and alignment marks on said mask relative to a fixedposition of said stage assembly and for calculating a distance to movesaid stage assembly in said first direction, a distance to move saidstage assembly in said second direction, and a angle to rotate saidstage assembly through in order to align said center points of saidalignment targets with said center points of said alignment marks. 18.The system of claim 1, wherein said alignment fixture comprises: a topring; a bottom ring having an inner lip for supporting an interiorregion of said substrate and an outer lip for supporting a peripheralregion of said mask, said inner lip higher than said outer lip;retaining posts fixed in and extending from said top ring and throughretaining post holes in said bottom ring; removable spring clipsengaging said retaining posts; wherein said top ring presses saidperipheral portion of said mask against said outer lip when saidalignment fixture is assembled.
 19. The system of claim 1, furtherincluding a computer for controlling and coordinating vacuum pressure tosaid means for holding said substrate, operation of said two or moremeans for temporarily affixing, operation of said means for controllingsaid means for temporarily affixing, operation of said means foraligning, and operation of said means for temporarily fastening.
 20. Thesystem of claim 1, wherein said mask is a metal mask.
 21. The system ofclaim 1, wherein said substrate is a semiconductor wafer.
 22. The systemof claim 1, wherein said mask is a solder bump evaporation mask.
 23. Asystem for of aligning a mask to a substrate comprising: an alignmentfixture for temporarily holding said mask and said substrate in fixedpositions relative to each other, said alignment ring having a top ringand a bottom ring; means for holding said substrate by a bottom surface,said means for holding said substrate protruding through an opening in atable and an opening in said alignment fixture, said means for holdingfixedly mounted on a stage assembly, said stage assembly moveable infirst and second directions and rotatable about an axis relative to saidtable; means for temporarily affixing said alignment fixture containingsaid mask and said substrate to said table; means for controlling saidmeans for temporarily affixing so as to generate a uniform force arounda perimeter of said alignment fixture to effectuate said temporarilyaffixing; means for aligning said mask to said substrate, said means foraligning controlling movement of said stage assembly in said first andsecond directions and rotation about said axis; and means fortemporarily fastening said alignment fixture together